This invention relates to electrical power transformers, particularly of the larger and largest sizes now made and used commercially, which normally use cellulosic electrical insulation, such as pressboard and Kraft paper, in a tank normally filled with transformer oil which impregnates the cellulosic material, the tank being closed or sealed as hermetically as is currently possible, to prevent the entrance of oxygen which has for a long time been considered harmful with respect to the transformer oil. The cellulosic insulation additionally serves to mechanically space electrically conducting parts of differing potentials within the casing, while providing increased electrical insulation strength as compared with an equally thick oil layer. The oil also functions as coolant to carry away heat unavoidably developed by the current-carrying components within the tank.
Such transformers, as currently made by reputable manufacturers, are in operation in many countries and they operate in a successful manner insofar as meeting the requirements currently demanded.
When such a transformer is first put into service, it is filled with transformer oil that is as clean and as free as possible from oxygen or other gases and water, the casing then being sealed as hermetically as is possible today. However, it is known that with time inevitable leakages will cause the oil to increase its oxygen content, and that other gases will accumulate in the tank. Also, the oil will become contaminated with water and solids.
Apparatus is commercially available for filtering and degassing and drying the oil, which can be connected to the transformer, the transformer oil being pumped through this apparatus which then filters the oil and subjects the oil to a high vacuum treatment for degassing and water removal. This apparatus, with a single pass of the oil, is said to be able to reduce the water content in the oil to not more than 5 parts per million (by weight), and the gas content in the oil to less than 0.1% by volume, while removing 99.9% of all particles larger than one micron.
Such an apparatus is used when putting a new transformer in service and possibly when the oil is changed in the transformer.
In spite of that, transformers deteriorate due to different aging processes and aging products.
Many scientific works on aging phenomena in power transformers have been published, and many investigations on the aging of transformers in service have been reported, but insofar as is known, these findings have not been linked together and no practical conclusions have been drawn. The best efforts made so far in the direction of slowing transformer deterioration has been to use the best means available to hermetically seal the transformer tank against the entrance of oxygen and moisture and to use antioxidants in the oil and in the cellulosic material.
During the operation of a sealed or closed transformer having the usual insulation and spacers of cellulosic material, such as pressboard and Kraft paper, and using usual transformer oil insulation, gases are evolved as deterioration products of the insulation materials. The production rate of these gases increases dramatically with increasing temperatures of the materials. This makes it possible to check the state of the transformer insulation by analyzing the gases existing in the transformer.
During development of this invention, it was found that transformer oil, aged at temperatures from 110.degree. C to 170.degree. C in a closed system, virtually ceases to produce hydrocarbon and carbon-oxide gases, when the oil contains less than 2,000 parts per million, by volume, of oxygen. At higher oxygen contents the oil produces substantial volumes of gases, independently of the oxygen content.
In the following, ppm is used to mean parts per million by volume per volume of oil at 0.degree. C and a pressure of 760 mm mercury.
It was further found by aging dry cellulosic transformer insulation in the substantial absence of oxygen, using temperatures up to 180.degree. C for several weeks, that carbon-oxides are produced by pyrolysis, but that substantially no hydrocarbons are produced. Hydrogen can be detected.
Since no hydrocarbons were found in the gases produced by the aging of the cellulosic material, which was Kraft paper, it was concluded that hydrocarbons found during gas analysis of working transformers, probably originated from deterioration of the transformer oil and not from the cellulosic material.
For verification of the above conclusion, experiments were performed where about 200 gm specimens of Kraft paper were heated in the range of 100 to 200.degree. C, with the paper in about 4000 gm of oil containing oxygen. As a result, it was found that the gases produced in the form of hydrocarbons were practically the same as for the transformer oil only, suggesting that the cellulosic insulation of a transformer, at least up to 200.degree. C, does not contribute to the production of hydrocarbons. However, the production rates of carbon-oxides was larger than indicated by the previously described work with the cellulosic material, per se, in a substantially oxygen-free atmosphere, suggesting that the oxygen content of the oil played some part in the production of the carbon-oxides. Inferentially there was the possibility that oxygen contents in the oil even below 2,000 ppm might have some effect on the aging of the cellulosic material when in the presence of the transformer oil, although the oil itself ceases to evolve hydrocarbons.
Therefore, two prolonged heat runs were performed on the same new transformer, this transformer having a rating as follows: 6.3 MVA, 22.5 .+-. 8.67 percent 11.5 - 6.65 kV, oil weight 5200 kg. This transformer was of a normal construction having coolers and a conservator, and also, of course, was of the sealed or closed type designed to operate under hermetically sealed conditions to the fullest extent possible by the present state of the transformer art.
For a first heat run, the transformer and its coolers were carefully dried and filled with thoroughly degassified oil under vacuum, the conservator being initially completely filled with degassified and substantially oxygen-free oil. The oil in the conservator was then partly replaced by nitrogen. Instead of the usual short tubing between the transformer tank and the conservator, a 10 m long hose with a loop was installed as a replacement for the short tubing, with the intent of minimizing possible transfer of oxygen to the oil in the tank, from the conservator. By closing some of the coolers the transformer was operated at a top oil temperature of 90.degree. C and there held as constant as possible throughout a heat run of 55 hours. Oil samples were taken several times a day for analysis, and it was found that the total gas content of the samples was around 0.3%, the gas being mainly nitrogen and the oxygen content below 300 ppm.
The second heat run was made under the same time and temperature conditions on the same transformer but with the oil containing 30,000 ppm oxygen to determine the effects of oxygen on the production of carbon-oxides and, hydrogen gas found to result from the pyrolysis of the cellulosic material per se, in the substantial absence of oxygen. Even at this 30,000 ppm oxygen content, no hydrocarbon gases were developed, but the development of carbon-oxide gases was increased substantially compared with the first run.
The foregoing led to the discovery that if the oxygen content of the transformer oil was kept below 300 ppm, the rate of the production of the carbon-oxides was radically reduced, whereas operation of the transformer with oil having higher values of oxygen produced a sharp increase in the production rate of these gases.
A change from the transformer oil containing 30,000 ppm of oxygen to one containing less than 300 ppm was found to reduce the production rate of carbon-oxides radically. CO.sub.2 is reduced by a factor of at least 5 and CO by a factor of 10. This produces the inevitable conclusion that the life of a transformer of the type described can be increased by a factor of 5 or, and this might possibly be more important, the operating temperature of the transformer can be increased by about 25.degree. C with a life expectancy unchanged over that which would be otherwise considered normal in the case of prior art transformer operation.
The transformer oil in an operating transformer can be kept with an oxygen content under 300 ppm by connection with and operation of the prior art filtering and degassifying apparatus, continuously or substantially continuously during the transformer operation. Therefore, a transformer of the type described preferably should be built to include an integrated filtration and degassification unit.